Striped crayfish snakes (Regina alleni) undergo a dietary shift from dragonfly larvae to crayfish during ontogeny. Godley (1980) suggested that this shift is attributable to crayfish availability rather than an initial preference for dragonfly larvae. We experimentally tested this hypothesis by measuring the chemosensory response of newborn snakes to prey odors at 2 ages and also after they were fed on either dragonfly larvae or crayfish. The results show that R. alleni respond equally to dragonfly larvae, hard crayfish, and soft crayfish before feeding experience. We also show that the maintenance of this preference over fish and control stimuli is subsequently determined by the prey type encountered, through an unusual interaction. Snakes fed dragonfly larvae increased their chemosensory response to both dragonfly larvae and crayfish, whereas snakes fed crayfish increased their response only to crayfish. Our study demonstrates that innate chemosensory responses to prey can be modified by prey availability and that they do not necessarily result from maturation alone. Such plasticity has adaptive value to newborn animals that must fend for themselves from birth and respond to changing environmental conditions.
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